The present invention is related to an electrolytic capacitor. More specifically the present invention is related to an electrolytic capacitor comprising intrinsically conductive polymeric cathode layers capable of achieving high break down voltages which were not previously available with polymeric cathode layers.
Solid electrolytic capacitors with intrinsically conductive polymers as the cathode material have been widely used in the electronics industry due to their advantageous low equivalent series resistance (ESR) and “non-burning/non-ignition” failure mode. Intrinsically conductive polymer, more commonly known as conductive polymer, is electrically conductive in the molecular level, in other words, a single molecule (a polymer chain) of this type of polymer is conductive, which distinguishes itself from other groups of polymeric materials whose electrical conductivity is imported from the presence of foreign conductive particles. The example of the latter is polyester (non-conductive) filled with carbon back (conductive particles). The intrinsically conducting polymer can exist in many physical forms including solid, solution, and liquid dispersion.
The backbone of a conductive polymer consists of a conjugated bonding structure. The polymer can exist in two general states, an undoped, non-conductive state, and a doped, conductive state. In the doped state, the polymer is conductive but of poor processibility due to a high degree of conjugation along the polymer chain, while in its undoped form, the same polymer loses its conductivity but can be processed more easily because it is more soluble. When doped, the polymer incorporates anionic moieties as constituents on its positively charged backbone. In order to achieve high conductivity, the conductive polymers used in the capacitor must be in doped form after the completion of process, although during the process, the polymer can be undoped/doped to achieve certain process advantages.
Various types of conductive polymers including polypyrrole, polyaniline, and polyethyldioxythiophene are applied to the Ta capacitors. The major drawback of conductive polymer capacitors, regardless of the types of conductive polymers employed, is their relatively low working voltage compared to their MnO2 counterparts. The polymer capacitors have reliability issues, to varying degrees, when the voltage rating exceeds 25V. This is believed to be caused by the relatively poor dielectric-polymer interface, which has poor “self-healing” capability. The ability to withstand high voltage can be best characterized by the breakdown voltage (BDV) of the capacitors. Higher BDV corresponds with better reliability. For reasons which were previously unknown the break down voltage of capacitors comprising conductive polymers has been limited to about 55V thereby leading to a capacitor which can only be rated for use at about 25V. This limitation has thwarted efforts to use conductive polymers more extensively.
In a manufacturing process to produce conductive polymer Ta capacitors, Ta powder is mechanically pressed to Ta metal pellets, which are subsequently sintered at high temperature under vacuum. The sintered anodes are then anodized in a liquid electrolyte at elevated temperature to form a cohesive dielectric layer (Ta2O5) on the anode surface. Increasing formation voltage increases the dielectric thickness, which determines the maximum voltage the anodes can withstand. The anodized anodes are then processed through multiple dipping cycles, which deposit conductive polymer onto the anode dielectric surface via in situ polymerization reactions. In the early stage of the coating process, reactant solutions are designed to gain easy access to the full surface area of the porous anodes without blocking the pores; while in the late stages, the coating process is designed to form a thick, dense external polymer layer, in order to protect the anodes from potential mechanical damages in the post polymerization process and from direct contact with graphite and silver layer, which are applied consequently onto the polymer coated anodes. The graphite/silver coated anodes are then assembled, molded, and tested to complete the manufacturing process.
For Ta capacitors, their rating voltage, or the working voltage allowed for reliable operation, is dictated by the dielectric thickness, which is controlled by the formation voltage. Increasing formation voltage increases the dielectric thickness. It is estimated that for every volt applied during the dielectric formation process, about 1.7˜2 nm of dielectric is formed on the surface. For a given anode, increasing dielectric thickness is at a cost of capacitance loss since the anode capacitance is inversely proportional to dielectric thickness. It is a common practice for Ta capacitor manufacturers that the formation voltage of anodes is 2.5 to 4 times higher than the anode rated voltage to ensure high reliability during applications. For example, a 10V rated capacitor often employs an anode formed at 30V.
The major drawback of the existing conductive polymer technology is its limited ability to produce high voltage Ta capacitors. This is well illustrated in the products currently available in the market. For example, conductive polymer capacitors only can be rated at no more than 25V while their MnO2 counterparts have ratings of greater than 50V.
A plot of the BDV versus the formation voltage for a wide range of Ta capacitors including both polymer (polyethyldioxythiophene, or PEDOT) and MnO2 based capacitors is shown in FIG. 1.
As shown FIG. 1, in the low formation voltage region (≦30V), the BDV of both polymer and MnO2 capacitors are close to the anode formation voltages. However, there is a trend of divergence in terms of BDV between MnO2 and polymer capacitors as formation voltage increases from about 80V to 200V. In this range, while the BDV of MnO2 parts still increase with the increasing formation voltage, the BDV of polymer parts shows a mostly flat pattern, which seems to hit a limit of about 50V and this limit is almost unaffected by the increasing formation voltage. Increasing dielectric thickness, which is the most important and commonly used approach to make high voltage capacitors, is virtually ineffective for making high voltage polymer capacitors beyond about 25V ratings. Due to this phenomenon the Ta industry has had difficulty producing reliable conducting polymer capacitors for use above 25 V. A 35V rated capacitor, for example, would require a BDV of far greater than 50V to ensure its long term reliability (e.g. 35V rated MnO2 parts have an average BDV of 115V). This also suggests that the dielectric in polymer capacitors degrades significantly under high electric field.
It is generally believed in the art that two major factors cause degradation of the dielectric. One factor is the presence of ionic species such as Fe+2 and Fe+3. Fe+3 is the reactant (oxidant) of the polymerization reaction while Fe+2 is the byproduct. It has been widely believed that the presence of such ionic species on the dielectric/polymer interface may cause dielectric breakdown under high electric field due to ionic migration. The other factor is based on the properties of conductive polymers themselves. Polymers are redox active meaning they can be reduced or oxidized. It has been widely believed that their redox properties influence the BDV of the dielectric/polymer interface.
We have investigated the impact of Fe+2 and Fe+3 on BDV by washing conductive polymer coated anodes in 1 wt % toluenesulfonic acid (TSA) solution for 24 hours. As shown in Table 1, while washing in TSA solution was effective in significantly reducing Fe residue content in the polymer covered anodes, the BDV of these washed anodes appeared unaffected relative to the control. Ammonium persulfate, or (NH4)2S2O8, was also employed to replace Fe+3 as the oxidant, thereby eliminating any presence of Fe ions in the polymer. No improvement in BDV was observed.
TABLE 1Fe content, ppmBDV, VControl9055Washed655(NH4)2S2O8 as oxidant—45
In further efforts to elucidate the impact of conductive polymer on BDV, the polymer was doped with iron (III) dodecylbenezenesulfnate (DBS) instead of iron (III) toluenebenezenesulfonate (Fe(TS)3). This approach produced polymers doped with DBS instead of toluenebenezenesulfonate. Likewise, (NH4)2S2O8 was used as the oxidant and the sodium salt of polystyrenesulfonate (PSS) was used as the dopant. In addition, polypyrrole (PPy) was pre-coated on the anodes followed by coating with PEDOT. This was designed to introduce a conductive polymer with redox properties that was different from those of PEDOT.
As shown in Table 2, the BDV of polymer anodes appeared unaffected by modifications in polymer properties or by altering the types of dopants in the conductive polymer
TABLE 2BDV, VControl (TS as dopant)45PSS as dopant40DBS as dopant43PPy as precoat48
There has been a long standing desire in the art to provide a capacitor comprising a conducting polymeric cathode suitable for use at higher rated voltages. Artisans have long considered the iron content and polymer properties to be critical yet this has still led to failure. Through diligent research the present inventors have achieved what was previously not considered feasible.